The invention relates to a shaving head for a dry-shaving apparatus which is adapted to be driven by an electric motor and is of the type in which a cutting structure provided with a plurality of cutting edges is arranged to be driven into reciprocating translation by means of a driving member coupled to a driving motor. The cutting edges cooperate with a thin resilient curved shear foil which is stretched across the cutting structure and is provided with hair entrance openings. Also there is provided between the cutting structure and the driving element a driving coupling such that the cutting structure, at least within certain limits, is tiltable about a tilting axis which extends parallel to the direction of the said translation.
Various embodiments of dry-shaving apparatus provided with a shaving head of the type described are known. For example a dry-shaving apparatus is known in which the cutting structure is hingedly coupled to the driving member. The pivotal axis extends in the direction of movement of the cutting structure. Furthermore dry-shaving apparatus are known which have a universal drive coupling, for example in that the end of a driving member coupled to a vibrator motor is spherical and is clampingly connected to resilient parts of the cutting structure. Such a universal drive coupling permits tilting of the cutting structure not only about a tilting axis which extends in the direction of movement, but also about two tilting axes at right angles thereto.
When during the use of the dry-shaving apparatus the shear foil of a shaving head of the type described is pressed against the parts of the skin to be shaved, the cutting structure may perform not only a reciprocating motion but also tilting movements. The tilting movements will be of a periodic nature having a frequency equal to that of the reciprocating motion of the cutting structure.
Owing to the loading of the shear foil, forces are exerted on the cutting structure via this foil. Some of these forces are due to deformations of the shear foil. Under the action of all these forces the cutting structure is tilted, because tilting moments with respect to the tilting axis are produced. In the known shaving heads the direction of tilting of the cutting structure is such that the tilting produces an increase in the tilting moments. Hence, with respect to tilting, the equilibrium of the cutting structure is unstable, so that during use, the cutting structure will always tend to continue tilting in the instantaneous direction of tilting. Obviously limits must be set to the tilting; in the known dry-shaving apparatus this is achieved in that at a given instant further tilting is prevented by fixed parts of the apparatus or by the driving member itself, or in that the thrust springs which keep the cutting structure pressed against the shear foil counteract further tilting in a manner such as to produce a state of equilibrium.
In these cases the shaving effect of the shaving apparatus will be adversely affected, for owing to the tilting of the cutting structure the cutting action of the cutters and hence the shaving effect will decrease, because the cutting edges move to the unloaded area of the shear foil and hence out of the area at which the skin of the user presses against the shear foil. This is why directions for use of known dry-shaving apparatus contain the instruction for the user to hold the apparatus pressed against his face at right angles to the area being shaved. Since such manipulation of the dry-shaving apparatus is found to be tiring owing to the raised position in which the user must hold his arm for a comparatively long time, many users do not obey this instruction, with a consequent decrease in shaving effect.
Furthermore, owing to the large tilting moment which may eventually be produced, the frictional forces which are produced in the driving coupling between the driving member and the cutting structure and which do not act in the driving direction and the ensuing friction moments may assume values such that the cutting structure periodically, particularly at the ends of its stroke, will be locally moved away from the shear foil. This behaviour of the cutting structure contributes to a painful action of the shaving apparatus, because a tensile force is exerted on the hairs which are pinched between the shear foil and the cutting edges.